Problem: The scale on a map is 3cm : 3km. If two cities are 12cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 3cm on the map corresponds to an actual distance of 3km. A distance of 12cm on the map is the same as 4 $\cdot$ 3cm. The actual distance between the two cities is 4 $\cdot$ 3km, or 12km.